ABSTRACT

Mercury contamination at historic gold mining sites represents a potential
risk to human health and the environment. Elemental mercury (quicksilver) was
used extensively for the recovery of gold at both placer and hardrock mines
throughout the western United States. In placer mine operations, loss of mercury
during gold recovery was reported to be as high as 30 percent. In the Dutch
Flat mining district located in the Sierra Nevada region of California, placer
mines processed more than 100,000,000 cubic yards of gold-bearing gravel. The
placer ore was washed through mercury-charged ground sluices and drainage tunnels
from 1857 to about 1900, during which time many thousands of pounds of mercury
were released into the environment.

Mine waters sampled in 1998 had total unfiltered
mercury concentrations ranging from 40 ng/L (nanograms per liter) to 10,400
ng/L, concentrations of unfiltered methyl mercury ranged from 0.01 ng/L to 1.12
ng/L. Mercury concentrations in sluice-box sediments ranged from 600 µg/g (micrograms
per gram) to 26,000 µg/g, which is in excess of applicable hazardous waste criteria
(20 µg/g). These concentrations indicate that hundreds to thousands of pounds
of mercury may remain at sites affected by hydraulic placer-gold mining. Elevated
mercury concentrations have been detected previously in fish and invertebrate
tissues downstream of the placer mines. Extensive transport of remobilized placer
sediments in the Bear River and other Sierra Nevada watersheds has been well
documented. Previous studies in the northwestern Sierra Nevada have shown that
the highest average levels of mercury bioaccumulation occur in the Bear and
South Fork Yuba River watersheds; this study has demonstrated a positive correlation
of mercury bioaccumulation with intensity of hydraulic gravel mining.